class_docstr: This stub class exists to return the appropriate subplot
  class when __call__-ed with an axes class. This is purely to
  allow Pickling of Axes and Subplots.
code: def __call__(self, axes_class):
      # create a dummy object instance
      subplot_instance = _PicklableSubplotClassConstructor()
      subplot_class = subplot_class_factory(axes_class)
      # update the class to the desired subplot class
      subplot_instance.__class__ = subplot_class
      return subplot_instance
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import six
  from six.moves import map
  from matplotlib.gridspec import GridSpec, SubplotSpec
  from matplotlib import docstring
  import matplotlib.artist
  from matplotlib.axes._axes import Axes
  import matplotlib._layoutbox
  import warnings
  from matplotlib.cbook import mplDeprecation
label_desc: Use image node_id 1 for calling the _PicklableSubplotClassConstructor obj's underlying member method code with example usage: obj.__call__(axes_class) and returns: subplot_instance